The rare earth element (REE) geochemistry of various phases from the active TAG hydrothermal mound has been examined and related to their mineralogy and fluid chemistry. The mound deposits range from black and white smoker chimneys, massive anhydrite/sulphide mixtures, oxides, and ochres. All phases, except black smoker chimney anhydrite, demonstrate a positive Eu anomaly when normalised to chondrite REE values. REE substitution into sulphide and sulphate phases appears to be strongly influenced by crystallographic control for all REE other than Eu. Precipitation of anhydrite within the TAG mound is the major mechanism for removal of REE during mound circulation and 0.15-0.35 g anhydrite is inferred to precipitate from every kg of fluid venting from the white smoker chimneys. Oxides from the mound fall into three different categories with distinct REE patterns: oxide rims on sulphides, atacamite-bearing oxides, and silica-rich Fe-oxides and ochres. The oxide rim phases contain sulphide and seawater derived REEs whereas the atacamite-bearing oxides and the ochreous material exhibit no seawater signature which suggests precipitation from, or alteration by, a modified hydrothermal fluid.

Rare earth element geochemistry of hydrothermal deposits from the active TAG Mound, 26°N Mid-Atlantic Ridge

High-temperature partitioning of the stable isotopes of rock-forming elements like Mg, Si, Fe, Ni and others are useful new tools in geochemistry and cosmochemistry. Understanding the fundamental driving forces for equilibrium inter-mineral fractionation comes from basic crystal chemistry and is invaluable for interpreting data from natural systems. Both charge and coordination number are key factors affecting bond length and bond stiffness and therefore the relative proclivity of a mineral phase for concentrating heavy or light isotopes. Quantitative interpretation of the plethora of new data relies on refinements of equilibrium fractionation factors through a feedback between crystal chemical reasoning, ab initio predictions, experiments, and analyses of well-characterized natural samples. This multifaceted approach is leading to a rapid rate of discovery using non-traditional stable isotopes in high temperature systems. For example, open-system mass transfer in the mantle is becoming increasingly evident from departures from equilibrium Mg and Fe isotope ratio partitioning between minerals, and differences in isotope ratios between bulk silicate Earth and meteorites are elucidating the conditions for Earth's core formation quantitatively. These applications rely critically on accurate equilibrium fractionation factors.

High-Temperature Equilibrium Isotope Fractionation of Non-Traditional Stable Isotopes: Experiments, Theory, and Applications (Invited)

Highly siderophile element depletion in the Moon

186Os–187Os and highly siderophile element abundance systematics of the mantle revealed by abyssal peridotites and Os-rich alloys

Hotspot volcanic rocks are formed under conditions that differ from conventional plate tectonic boundary magmatic processes and are compositionally distinct from mid-oceanic ridge basalts. Hotspot volcanic rocks include – but are not limited to – ocean island basalts (OIB), continental flood basalts (CFB), komatiites, oceanic plateau and some intraplate alkaline volcanic rocks. Studies of the highly siderophile element (HSE) geochemistry of hotspot volcanic rocks have provided new perspectives into mantle convection, mantle heterogeneity, core-mantle interactions, crustal and mantle lithospheric recycling, melting processes and crust-mantle interactions. The HSE, comprising Os, Ir, Ru, Rh, Pt, Pd, Re and Au, have strong affinities for metal and sulphide relative to silicate. These elements also have variable partitioning behaviour between highly compatible Os, Ir, Ru and Rh relative to compatible Pt and Pd and to moderately incompatible Re and Au during melting and crystallisation. This exceptional geochemical behaviour, combined with the existence of the long-lived 187 Re- 187 Os and 190 Pt- 186 Os decay systems embedded within the HSE, make these elements powerful tracers of processes acting on magmas and their volcanic products. The HSE can be utilised to understand sub-aerial volcanic degassing and crustal assimilation processes in hotspot volcanic rocks such as CFB and OIB, as well as for quantitative assessment of fractional crystallisation. Mantle melting studies have highlighted the strong control of sulphide in the mantle prior to exhaustion of S and generation of Os ± Ir ± Ru metal alloys at ~ > 25% partial melting; a behaviour of the HSE that is fundamental to understanding terrestrial hotspot volcanism. Perhaps the most exciting utility of the HSE, however, lies in their ability to reveal both short- and long-term fractionation processes acting on hotspot volcanic sources from inter-element HSE fractionations and 187 Os/ 188 Os- 186 Os/ 188 Os systematics. The growing database for HSE abundances and 187 Os/ 188 Os in hotspot volcanic rocks is consistent with their generation from a heterogeneous upper mantle generated by melt differentiation and recycling of crust and mantle lithosphere during plate subduction. 187 Os/ 188 Os variations (ratios up to 0.175) in high-Os abundance (> 50 ppt) HIMU (high 238 U/ 206 Pb) OIB indicate high long-term (> 1 to 2 Ga) Re/Os, supporting models for recycled oceanic lithosphere in the source of these volcanic rocks. In contrast, and despite elevated 87 Sr/ 86 Sr and low 143 Nd/ 144 Nd, enriched mantle (EM) OIB can have 187 Os/ 188 Os that dominantly reflect contributions from peridotite with only minor contributions from recycled sediment or continental crust and/or lithospheric mantle materials. The HSE provide geochemical evidence for how lithological and chemical heterogeneities are sampled within the mantle. Modeling of HSE abundances and Os isotopes show that large apparent recycled contributions (50% to 90%) in some OIB can be explained by the preferential melting of volumetrically minor ( 186 Os/ 188 Os data for hotspot volcanism are limited, but the combined variations in long-term Re/Os and Pt/Os retained in some mantle sources may reflect either the long-term fractionation of Re and Pt from Os between the inner and outer core, or ancient sulphide segregation and lithological variations in the mantle caused by convection but unrelated to core-mantle interaction. Either mode-of-origin is important for a firmer understanding of processes occurring in Earth, and both suggest that some hotspot volcanic mantle sources have been isolated and have evolved with supra-chondritic Pt/Os over > 2 to 3 Ga time-scales. Study of the HSE in hotspot volcanic rocks from Solar System bodies also informs on planetary-scale processes, indicating that Earth, the Moon, Mars and fully differentiated asteroids all have HSE abundances in their mantles that are higher than expected from low-pressure metal-silicate partitioning. Furthermore, the HSE are in broadly chondritic-relative abundances for these planetary bodies, at ~ 0.0002 (Moon), to ~ 0.007 (Mars), to ~ 0.009 (Earth) × carbonaceous chondrite Ivuna (CI) composition. The timing of addition of the HSE to planetary bodies preserved in their magmas and volcanic products is consistent with Solar-System-wide late accretion no later than 3.8 Ga for Earth, and even earlier based on evidence from the Moon (~ 4.4 Ga), Mars (~ 4.5 Ga) and asteroids (> 4.56 Ga).

Hotspot volcanism and highly siderophile elements

Neodymium-142 isotope data from young Reunion Island volcanic rocks reflect the effects of geological processes that occurred more than four billion years ago, showing that the deep mantle may preserve geochemical signatures of the primordial Earth. Certain volcanic hotspots are thought to sample sections of Earth's deep interior that were formed more than two billion years ago, with some having witnessed differentiation events—the separation of planetary materials into chemically distinct regions, such as the core and mantle—that occurred within the first 50 million years of Earth's history. Bradley Peters and colleagues report neodymium isotopic data from Reunion Island volcanic rocks. The data correlate with helium isotopic data, suggesting parallel behaviour of the isotopic systems during silicate differentiation, perhaps as early as 4.39 billion years ago. They conclude that Reunion magmas tap an unusually ancient, primitive source compared to other volcanic hotspots, which could offer a window into the formation and preservation of ancient heterogeneities in Earth's interior. Active volcanic hotspots can tap into domains in Earth’s deep interior that were formed more than two billion years ago1,2. High-precision data on variability in tungsten isotopes have shown that some of these domains resulted from differentiation events that occurred within the first fifty million years of Earth history3,4. However, it has not proved easy to resolve analogous variability in neodymium isotope compositions that would track regions of Earth’s interior whose composition was established by events occurring within roughly the first five hundred million years of Earth history5,6. Here we report 142Nd/144Nd ratios for Reunion Island igneous rocks, some of which are resolvably either higher or lower than the ratios in modern upper-mantle domains. We also find that Reunion 142Nd/144Nd ratios correlate with helium-isotope ratios (3He/4He), suggesting parallel behaviour of these isotopic systems during very early silicate differentiation, perhaps as early as 4.39 billion years ago. The range of 142Nd/144Nd ratios in Reunion basalts is inconsistent with a single-stage differentiation process, and instead requires mixing of a conjugate melt and residue formed in at least one melting event during the Hadean eon, 4.56 billion to 4 billion years ago. Efficient post-Hadean mixing nearly erased the ancient, anomalous 142Nd/144Nd signatures, and produced the relatively homogeneous 143Nd/144Nd composition that is characteristic of Reunion basalts. Our results show that Reunion magmas tap into a particularly ancient, primitive source compared with other volcanic hotspots7,8,9,10, offering insight into the formation and preservation of ancient heterogeneities in Earth’s interior.

Hadean silicate differentiation preserved by anomalous 142Nd/144Nd ratios in the Réunion hotspot source.

Oxygen isotope systematics of South African olivine melilitites and implications for HIMU mantle reservoirs

Early mantle heterogeneities in the Réunion hotspot source inferred from highly siderophile elements in cumulate xenoliths

The sensitivity of trace element concentrations to processes governing solid-melt interactions has made them valuable tools for tracing the effects of partial melting, fractional crystallization, metasomatism, and similar processes on the composition of a parental melt. Recent studies of ocean island basalts (OIB) have sought to correlate Ti, Ta, and Nb (TITAN) anomalies to isotopic tracers, such as 3He/4He and 187Os/188Os ratios, which may trace primordial deep mantle sources. A new compilation of global OIB trace element abundance data indicates that positive TITAN anomalies, though statistically pervasive features of OIB, may not be compositional features of their mantle sources. OIB show a range of Ti (Ti/Ti* = 0.28–2.35), Ta (Ta/Ta* = 0.11–93.4), and Nb (Nb/Nb* = 0.13–17.8) anomalies that show negligible correlations with 3He/4He ratios, indicating that TITAN anomalies are not derived from the less-degassed mantle source traced by high-3He/4He. Positive TITAN anomalies can be modeled using variable degrees (0.1–10%) of nonmodal batch partial melting of garnet-spinel lherzolite at temperatures and pressures considered typical for OIB petrogenesis, and subjecting this partial melt to fractional crystallization and assimilation of mid-ocean ridge basalt-like crust (AFC). Correlations of TITAN anomalies with modal abundances of olivine and clinopyroxene in porphyritic Canary Islands lavas provide empirical support for this process and indicate that high abundances of these phases in OIB may create misleading trace element anomalies on primitive mantle-normalized spider diagrams. Because partial melting and AFC are common to all mantle-derived magmas, caution should be used when attributing TITAN anomalies to direct sampling of recycled or deep mantle sources by hotspots.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2014GC005506

Bradley J. Peters

Papers

Hadean silicate differentiation preserved by anomalous 142Nd/144Nd ratios in the Réunion hotspot source.

Oxygen isotope systematics of South African olivine melilitites and implications for HIMU mantle reservoirs

Early mantle heterogeneities in the Réunion hotspot source inferred from highly siderophile elements in cumulate xenoliths

Assessment of relative Ti, Ta, and Nb (TITAN) enrichments in ocean island basalts

Combined Lithophile-Siderophile Isotopic Constraints on Hadean Processes Preserved in Ocean Island Basalt Sources